feat(child_process): support inline execFile callback options

[ctate]

Summary

• Lower execFile(file, args, { encoding: "utf8", maxBuffer: N }, callback) through the existing asynchronous child process runtime.
• Accept only literal UTF-8 encoding and side-effect-free numeric literals or literal arithmetic for maxBuffer. Other callback options retain explicit diagnostics.
• Cover successful and failing children in the differential corpus, update the diagnostic snapshot, and regenerate the surface manifest.

The native capture grows without enforcing maxBuffer, consistent with the existing synchronous and promisified capture paths.

Validation

• Compiler build and focused C/LLVM differential tests pass after rebasing onto main.
• Diagnostic snapshot and pnpm node-compat:check pass.
• pnpm test:ts7 passes the full parity sweep (1,434 entries) after recording the new corpus fixture's baseline.
• pnpm test:sandbox: all 16 remote shards passed. Four local external lanes failed in the Vercel CLI network-jail preflight on Node 26.4.0: the unmodified CLI 56.2.0 first connects to sentry.io instead of api.vercel.com. The same isolated probe reaches api.vercel.com first on Node 24.15.0. That suite and the sandbox harness are unchanged in this PR.

[vercel]

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Project	Deployment	Actions	Updated
scriptc	Ready	Preview, v0	Sep 23, 2026 5:02am UTC